Hi i have purchased a yamaha xvs 650 vstar classic 2003 model fitted with BUB exhaust pipes and i am getting poping and back fireing when decellarating and have been told the carby may need rejetting,could this be the problem or is there something else to look at, thanks peter

If the mechanic did not replace the exhaust pipe gaskets in the head or on any connections between the head pipes and the mufflers , the air leaks could be igniting unburned fuel in the exhaust , causing the popping and back firing. Try replacing any gaskets in the exhaust system.
If you have a flat spot or hesitation when accelerating than you probably also need to rejet the carburetors. Bigger main jets should fix that condition. A jet kit may also contain differently tapered needles for the slides to help compensate for the change in the acceleration curve.